I bought this book several years ago, back when I qualified for an educator discount and could get paperbacks from Random House for $3 apiece. Whenever you see me review a book that seems out of my usual oeuvre (I try not to have a usual oeuvre, but my prejudices seep out), chances are that it dates from this era – either that or from some library book sale. I decided the time had come to read this book when I was helping a student with her science homework and found that the chart in her textbook of all the planets ended with Neptune. Seeing the updated chart and talking with my student, who had no idea that there had ever been a planet called Pluto, made me care about this change in the solar system more than vaguely remembering seeing a headline several years ago, most likely when I was busy thinking about other things.

This book is a light, quick read meant for laypeople. Mike Brown is a CalTech professor of astronomy who discovered several “objects” in the farthest reaches of the solar system during the early years of the 21st century. Around this same time, other astronomers first saw the Kuiper belt – a second asteroid belt past Neptune that was named after the astronomer who first theorized that it existed. The Kuiper belt is important to the demotion of Pluto mentioned in the title.

Brown begins the book with a quick trip through the history of our understanding of the solar system, and if there’s one thing I love, it’s intellectual history. He traces the word ‘planet’ back to ancient Greece, where it was used to mean any object that moves through the sky rather than staying in place. Under that definition, the sun and the moon were planets, while the earth was not. The ancient Greeks counted seven planets – the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn – all of which could be seen with the naked eye. Brown leaps forward to Galileo’s telescope and the rapid-succession discovery of Uranus, Neptune, and a series of smaller “planets” that would later be reclassified as asteroids, a term that did not yet exist in these early years of telescope development. The word ‘planet’ was called into question once again and eventually defined not only as an object that traveled through the sky but also as an object that orbited the sun. The earth became a planet. The sun was classified as a star. The moon was classified as a satellite, and astronomers discovered similar satellites around most of the other planets. Initially declared to be planets and given names (because they do travel through the sky and orbit the sun), the asteroids were downgraded because the word ‘planet’ was redefined to mean something that orbits the sun on its own. Since the asteroids all share an orbit, they are not planets.

The short version of Pluto’s demotion (its new status is “dwarf planet”) follows a similar trajectory. During the years Mike Brown covers in this book, astronomers were finding new objects in the far reaches of space in a way that mimicked the discovery of the asteroid belt. These objects were later demonstrated to be the Kuiper belt. Some of these objects were extremely far out in space – hundreds of times as far as Pluto – and some of them had funky orbits that didn’t resemble the orbits of any known planets. Astronomers were once again faced with the question of how to define “planet.” Size came into question. Much smaller than the other eight planets, Pluto was popularly seen as not just the smallest planet but as the smallest possible planet. Whenever a newly discovered object was deemed to be smaller than Pluto, astronomers grumbled that it couldn’t possibly be a planet – in spite of the fact that “all planets must be the same size as or bigger than Pluto” is not a scientific statement, not at all.

Brown does a great job of portraying the role of the emotions in science. By this I mean not just the egos of individual scientists (“We recommend that you remain humble,” Brown was told by a bureaucrat at the International Astronomical Union after an object he found was – temporarily – deemed a planet) but the sentimentality that everyone – scientists and ordinary people – seems to attach to objects in space. When I first saw this book in Random House’s catalog, my first thought concerned the mnemonic I was taught in second grade to help me remember the planets: My Very Excellent Mother Just Served Us Nine Pizzas. What would happen to the mnemonic now that there was no more room in it for pizzas? I didn’t exactly obsess about it or anything, but I wondered. As it happens, Brown spends some time on that acronym in his book. It appears that there was a contingent out there who really did obsess about the acronym and were furious at Pluto’s removal from the planetary pantheon. Brown is both amused and bemused by the whole experience and tells the story with a light yet erudite touch.

It happened that during the worldwide debate about whether Pluto and other Kuiper belt objects would be classified as planets, Brown and his wife were also caring for their newborn. Brown conveys both the experience of falling in love with their infant daughter and the jarring experience of being yanked out of his time at home with his wife and newborn daughter to appear on TV at 5 am on the day the IAU announced its decision on Pluto. These sections of the book are humorous – sometimes in a “men are so clueless” sort of way, and who doesn’t love that? – and they also provide context to remind us that science is a human endeavor, always happening while the rest of life is going on, fraught with and at the mercy of human emotions. Ultimately, it is this focus on “real life” that makes this such an enjoyable book, both easy to read and enriching.